Numerous attempts are being made in the art of delignification of lignin-cellulose raw materials to meet the new requirements of regulations designed for protecting the environment. In particular, these regulations have incited papermakers to try to replace certain papermaking processes that are particularly polluting by processes able to produce pulps with equivalent characteristics without pollution.
This problem takes on a particular acuteness in regard to chemical pulp known as kraft, which exhibits characteristics on a very high level, but whose manufacturing process is particularly polluting. The kraft process, as it is well known, generates atmospheric pollution by release of foul-smelling sulfur compounds such as mercaptans, sulfur dioxide, hydrogen sulfide, or the like. Further, the resulting pulps require a subsequent thorough bleaching to achieve the level of whiteness necessary for specialized uses such as treatment with chlorine agents, which themselves are water pollutants.
To solve these problems, paper research has been particularly oriented toward the use of delignification agents that are not pollutants and, during their action on the lignin, do not cause pollutant products. Thus, cooking processes have been developed that take advantage of the delignification action of oxygen in an alkaline medium.
One such process consists essentially of a cooking of the lignin-cellulose material in two stages, separated by an intermediate grinding, successively comprising an alkaline cooking under pressure, an intermediate grinding in a disk apparatus, and finally a cooking under oxygen pressure in the presence of an alkaline solution. An example of the process is described in Canadian Pat. No. 895,756. Although this process yields a good quality pulp, without causing atmospheric pollution, the process has a certain number of drawbacks. On the one hand, the slight solubility of the oxygen requires steps to assure a rather homogeneous transfer between the gaseous phase, aqueous phase and cellulose fibers. The use of relatively high pressures, such as on the order of eight (8) to ten (10) bars with requirements up to twenty (20) bars, requires use of special burdensome equipment different from that used for the standard kraft process. To replace the kraft process by an oxygen process involves extremely heavy machinery and investments even for small and medium size units.
The quality of pulps obtained by the oxygen processes is slightly inferior to those of the kraft pulp, particularly because of the depolymerizing action of the oxygen on the cellulose. Most studies published on this subject tend to agree on recognizing this lower level of the mechanical characteristics and particularly the tear index. In this connection, there can be cited the publications of A. G. Jamieson, O. Samuelson, L. A. Smedman in Technical Association of Pulp & Paper Industry (TAPPI), Volume 58, No. 2, pp. 68-71; M. Saukkonen and I. Paulenius in TAPPI, Volume 58, No. 7, pp. 117-120; H. M. Chang, J. S. Gratzl, W. T. McKean, R. H. Reeves, and V. E. Stockman in TAPPI, Volume 59, No. 8, pp. 72-75.
In their most recent versions, the processes cited above have involved lower oxygen pressures and the corresponding reduction in machinery investments. Elimination of the intermediate grinding has been proposed in French Pat. No. 2,256,283, while improvement of the oxygen transfer has been proposed in French Pat. No. 2,220,620.
The investments necessary for the application of oxygen processing are still clearly greater than those of the kraft process, without improving the characteristics of the pulps being processed. Further, the lower level of the tearing index is particularly disadvantageous for using these pulps in the field of wrapping papers.
These differences explain why so far few of the prior art attempts have resulted in an industrial commercialization.
It has now been found that by practice of the present invention, the difficulties and disadvantages of prior art attempts to remove pollution contaminants from paper pulp processing have been overcome in a simple, highly efficient manner.